Boron occurs in nature principally in the form of borate minerals. These minerals are mined industrially as evaporate ores, such as borax and kernite. Elemental boron is used extensively as a dopant in the semiconductor industry, while boron compounds have a variety of industrial applications, including their use in sodium perborate bleaches and in fiberglass insulation. Thus, for example, boric acid is used in the manufacture of monofilament fiberglass usually referred to as textile fiberglass. Boron compounds also find use as high-strength, lightweight structural and refractory materials and in thermally stable glasses and ceramics, while boron-containing reagents are used as chemical intermediates in the synthesis of a variety of organic compounds.
Boron minerals are typically recovered through surface mining of evaporate mineral deposits such as those found at Boron, Calif. and Searles Lake, Calif., although methods are also known for recovering boron from geothermal brines. Unfortunately, many commercially promising evaporate deposits of boron minerals also contain significant amounts of lithium minerals. The presence of these lithium minerals represents a significant devaluation in the value of the deposits, due to the current lack of a commercially effective means for separating the lithium and boron values.
Moreover, conventional mining practices create a significant amount of waste materials that represent lost profits and raise environmental issues. These waste materials, often referred to as “tailings”, are the materials left over after the separation of the valuable fraction of an ore from the uneconomic fraction (or gangue). In a typical boric acid mining operation, these tailings are discharged to a tailings pond (in the case of liquid tailings mixtures), or to tailings piles (in the case of tailings solids).
Tailings represent a significant cost to a mining operation. Frequently, a mining company must employ expensive measures, such as dams or water barriers, to maintain a tailings pond and to prevent its contents from contaminating the local groundwater supply or environment. Indeed, the costs associated with a tailings pond are frequently the most significant environmental liability for a mining operation. Typically, environmentally responsible mining companies operating in jurisdictions with well developed mining regulations must account for the cost of the closure and rehabilitation of tailings ponds in their operations. In some jurisdictions, such as the province of Quebec, Canada, a closure plan for tailings ponds is required before mining activities may commence, and a financial deposit, amounting to 70% of the estimated rehabilitation costs, is also required.
Conventional technologies, such as ion exchange and solvent extraction, provide suitable extraction techniques for a variety of materials. However, these techniques are not suitable for the recovery of boric acid from boric acid plant tails, because the concentration of boric acid in the plant tails is typically too high for these approaches to be feasible. Moreover, the presence of solids in the plant tails significantly complicates ion exchange and solvent extraction, and adversely affects the economics of these approaches.
Some alternative methods have been developed in the art for separating lithium and boron minerals. For example, U.S. Pat. No. 5,236,491 (Duyvesteyn) discloses a method for the selective removal of boron from geothermal brines which involves passing the brines over a bed of an anionic resin. A pH value of about 4 to 5.5 is maintained to load the boron on the resin. The boron content is then stripped from the resin with an acid solution such as 1M hydrochloric or sulfuric acid. While this approach represents a notable advance in the art and may be useful in extracting boron values from relatively dilute brines, the resin must be in basic form before it can load boron. Hence, after the resin is stripped with an acidic solution, it must be subsequently washed with an alkaline solution in order to reactivate it. These requirements cause this technique to be cumbersome and uneconomical in practice in many applications, particularly if it is to be applied to feedstocks having higher concentrations of boron values.